1. Field of the Invention
This invention relates to the chemical arts. In particular, it relates to flexible polymeric foams.
2. Description of the Prior Art
Flexible polymeric foams have been known and produced commercially for many years. Until now, essentially all commercial foams have been either closed cell or open cell. Closed cell flexible foams have tended to be formed from polyolefins, while open cell flexible foams have most commonly been formed from polyurethanes.
Currently a sizeable market exists for both open cell and closed cell foam, particularly foam rods, as a backer material for sealants in joints in the highway, airport and building construction industries.
Foam rods are particularly useful in the repair of existing pavement. In new construction, the joint to be sealed is generally well formed with uniform sidewalls. In contrast, when repairing or reconditioning existing pavement, the joint can be very non-uniform, with numerous imperfections, called "spalls and ravels", which remain even if it has been resawed and cleaned. The purpose of the backer rod is to limit the penetration of the sealant into the bottom of the joint, so that the cured sealant will have optimum adhesion to the sidewall during subsequent expansion-contraction cycles. Adhesion of the sealant to the bottom of the joint is undesirable, since it places additional stress on the sidewall seal during expansion-contraction cycles leading to premature failure of the joint sealant integrity.
Both open and closed cell foam rods possess distinct disadvantages. The disadvantages of closed cell rods include their high compression resistance, low compression recovery and tendency to "outgas", if their cells are ruptured during installation. Their high compression resistnace and low compression recovery mandates a close match between rod size and joint size. This means that manufacturers must make and inventory a large number of sizes. Further, "outgassing" can cause bubbles to occur in the sealant, thereby reducing the sealant's effectiveness.
Open cell rods with their low compression resistance and high compression recovery can be supplied in fewer sizes and do not outgas, since there are no gas containing cells to rupture. However, open cell rods are hydrophilic and can wick moisture into the joint, thus causing water sensitive sealants to fail prematurely. The rough cellular surface of the rod can absorb sealant. Also, the nature of manufacture of open cell rods gives rise to several disadvantages. It can give rise to a dust problem. Further, open cell rods typically are not uniform and at regular intervals there are joints in the rods which exhibit different properties than the regular rod. Also, due to the materials of construction and methods of manufacture, open cell rods are inherently more expensive than closed cell rods.
The flexible, low density, ethylenic polymer foams of the invention overcome the problems surrounding conventional open or closed cell foams. The foams of the invention are extruded into simple shapes, exhibiting the low compression resistance, high compression recovery, and non-outgassing nature characteristic of open cell urethane foams, while possessing the integral or non-absorbing skin of closed cell foams.
Because rods formed from ethylenic polymers of the invention possess both low compression resistance and superior compression recovery, they are superior to existing backer rods when used in the repair or reconditioning of existing buildings or pavement. This is because they can easily conform to the rough, non-uniform joint sidewalls and stop leakage of the sealant material into the bottom of the joint.
The use of polymer blends and more particularly of low modulus ethylene copolymers to prepare low density polymeric foams is known in the art. It is disclosed in U.S. Pat. Nos. 4,101,467, 4,129,530 and 4,215,202. However, the foams are closed cell in nature.
U.S. Pat. No. 3,939,237 discloses using decomposition blowing agents to prepare partially open cell polyolefin foams. However, the foams obtained are quite high in density and are very thin, less than 3 mm.
U.S. Pat. No. 4,384,032 discloses relatively high density open cell polyolefin foams, which incorporate polystyrene or other incompatible polymers. Again, decomposition blowing agents are used either alone or in conjunction with a volatile blowing agent. The polystyrene, or other incompatible polymer, is crucial in providing the open cell nature of the product.
U.S. Pat. No. 3,379,802 discloses a process for producing open cell-closed cell low density foams by incorporating an elastomer into a molten olefinic polymer. The patent teaches that olefinic copolymers such as ethylene and ethyl acrylate and ethylene and vinyl acetate, which may have a low enough modulus to produce a soft foam in the unmodified state, cannot be extruded alone to produce a satisfactory foam, since the foams are dimensionally unstable and collapse upon extrusion.
U.S. Pat. No. 4,499,210 discloses a process for manufacturing a cross-linked polyethylene foam using a trifunctional monomer and silicone oil to produce an open cell structure. This composition is foamed subsequent to compounding, in a separate heat treatment.
Surprisingly, the ethylenic polymer foam of the invention is quite stable. Completely closed cell foams are notoriously unstable. Many patents, such as U.S. Pat. No. 4,359,539, and those cited therein, disclose the use of various compounds as blocking agents to dimensionally stabilize polyolefin foams. In contrast, the new foams, containing both open and closed cells, are stable, even when no blocking agents are included in the invention.